Apparatus for plasma-assisted high rate electron beam vaporization

ABSTRACT

It is known that improved coating properties can be obtained by plasma action in vacuum deposition, especially by vaporization. Substantially higher coating rates can be attained in vapor deposition, but, with high plasma densities, they result in excessive scattering of the electron beam and reduce the power density. According to the invention, a plasma source, preferably a hollow cathode are source, is arranged in the immediate vicinity of the substrate. Between the evaporator and the substrate there is a device for generating a magnetic field so that the region of high plasma density is separated from the evaporator and the electron beam by the magnetic field. The boundary field lines of this magnetic field run along an arc curving with respect to the substrate.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for the plasma-assisted, high rateelectron beam vaporization, preferably for the vapor deposition ofcoatings of compounds on strip-like and plate-like substrates. Typicalfields of application are the vaporizing of plastic films with barrierlayers for the packaging industry, with scratch-proof coatings forsun-shielding films or foils or with selectively reflecting coatings forheat-absorbing films or foils. Other uses occur in the coating ofmetallic strips for decorative purposes and for corrosion protection, aswell as in the coating of plastic or glass plates for obtainingscratch-proof or selectively reflecting surfaces. However, bettercoating characteristics also result from the additional plasma actionwhen vapor depositing metallic coatings on different substrates. Inparticular it is possible to obtain a higher packing density of thecoating and therefore improve mechanical, optical and electricalcharacteristics.

It is known that as a result of plasma action in vacuum coating improvedcoating characteristics can be obtained. For example, during sputtering,which naturally takes place under plasma action, in general bettercoating characteristics can be obtained than when vaporizing. However,the coating rates during sputtering are relatively low and areinadequate for many uses. It is possible to achieve higher rates byusing the vaporizing procedure.

It is also known to obtain similar coating characteristics to thoseobtained with sputtering in the case of plasma-assisted vaporizing.Simultaneously with the vapor-deposited material a plasma or the ionsextracted from a plasma are made to act on the substrate. The plasma isproduced either by ionizing the vapor zone by means of a separatelyproduced low energy electron beam or by igniting an independentdischarge between the vaporizer and the substrate (Schiller, S., Heisig,U., Panzer, S., Elekronenstrahltechnologie, VEB Verlag Technik, Berlin,1976, p.187ff and p.136ff).

The highest coating rates and therefore the lowest coating costs can beachieved by high rate electron beam vaporization. During the vapordeposition of metals coating rates up to 50 μm/s are obtained, but alsowhen vapor depositing compounds coating rates of a few μm/s can beobtained. In order to obtain at such high rates positive effects withregards to the coating characteristics by means of an additional plasmaaction, correspondingly high plasma densities are required on thesubstrate. High plasma densities can e.g. be produced with per se knownarc discharges. However, it has not hitherto been possible tosimultaneously bring into effect the high vaporizing rates and the highplasma densities on the substrate. The high plasma densities lead tosuch a pronounced spreading or scattering of the electron beam requiredfor vaporization, that it is not possible to achieve the power densitiesrequired for high rate vaporization.

It is also known to allow to burn in the vaporizer crucible a plasma arcdischarge in addition to the electron beam (Moil, E., Buhl, R., Pulker,H.K., Bergmann, E., Activated Reactive Ion Plating Surface and CoatingsTechnology 39/40(1989), pp.475-486). However, this additional vaporizerheating does not lead to the coating rates obtained with purely highrate electron beam vaporization. In addition, this procedure is onlysuitable for vaporizing conductive metals and not for vaporizinginsulating compounds. Moreover, the plasma action preferably takes placein the vicinity of the vaporizer crucible and not in the vicinity of thesubstrate.

SUMMARY OF THE INVENTION

The problem of the invention is to provide an apparatus forplasma-assisted, high rate electron beam vaporization making itpossible, whilst maintaining the high vaporization rates of high rateelectron beam vaporization to bring a high density plasma into action inthe vicinity of the substrate to be coated. In addition, an inadmissiblespreading and power density action of the electron beam by the necessaryhigh density plasma are to be avoided.

According to the invention this problem is solved by the features ofclaim 1. Further advantageous developments of the apparatus aredescribed in the subclaims.

As a result of the construction of the apparatus according to theinvention, i.e. the geometrical arrangement and construction of the polepieces or shoes of the magnetic field-generating device, the spatialassociation of the substrate, the hollow cathode arc source and themagnetic field, it is possible to adjust the spacing between the area ofthe highest plasma density and the substrate surface in such a way thatthere is a maximum plasma action on the substrate surface, but thesubstrate is not thermally damaged.

It has proved appropriate to select the maximum field strength of themagnetic field arranged in shield-like manner between the high plasmadensity area and the vaporizer, respectively electron beam between 1 and10 kA/m, preferably between 2 and 5 kA/m. Smaller field strengths canlead to an inadequate separation, whereas higher field strengths canlead to an undesired, strong deflection of the electron beam.

It is particularly advantageous for producing the magnetic field andguiding the electron beam to use a per se known magnetic trap. Thismagnetic trap, which is a magnetic system with pole pieces on eitherside of the vaporizer crucible, produces over the latter a generallyhorizontal magnetic field, through which the electrons backscattered onthe vaporizing material are kept away from the substrate to be coated.The electron beam is injected approximately perpendicular to thedirection of the magnetic field lines or against the field lines intothe horizontal magnetic field (East German patents 55 154, 64 107 and204 947). The combination of the magnetic plasma shielding according tothe invention and the magnetic trap for shielding against backscatteredelectrodes is particularly advantageous, because in the interest of amaximum plasma density on the substrate surface and due to the thermalaction on the substrate associated therewith, an additional stressing byelectrons backscattered on the vaporizer is undesired.

For producing the high plasma density area in the vicinity of thesubstrate surface it is possible to use with particular advantage hollowcathode arc sources. This makes it possible to attain charge carrierdensities adapted to the high rate electron beam vaporization between10¹¹ and 10¹² cm⁻³. The discharge arc follows the path of the fieldlines in the homogeneous marginal area of the magnetic field and strikesthe anode of the arc source arranged on the opposite pole piece. As aresult of the curvature of the field lines in the inhomogeneous marginalarea of the magnetic field the arc discharge acquires an outwardcurvature in the direction of the substrate. The discharge arc is guidedin clearly defined manner along the curved field lines, so that anuncontrollable arc path and therefore an undesired, direct impact of thedischarge arc on the substrate or other parts of the vaporizing chamberare avoided.

For the uniform coating of larger substrates it is appropriate to fanthe discharge arc through an alternating magnetic field at the locationof the arc exit from the hollow cathode into a plane parallel to thesubstrate surface. The deflection frequency must be chosen so high thateach point of the layer building up on the substrate is swept severaltimes by the discharge arc. It is advantageous in the case of very widesubstrates to arrange in juxtaposed manner several hollow cathodes alongone pole piece for producing the magnetic field and to place a common,correspondingly elongated anode in front of the facing pole piece. Hereagain it is appropriate to deflect the individual discharge arcs of thehollow cathode by alternating magnetic fields in fan-like mannerparallel to substrate surface, in order to obtain a high uniformity ofthe plasma zone perpendicular to the arc direction.

During the deposition of metal coatings a water-cooled copper plate hasproved advantageous as the anode for hollow cathode discharges. Whendepositing insulating layers by reactive vaporization of metals or bydirect vaporization of insulating compounds after a very short time theanode is covered with an insulating coating. It is appropriate in thiscase to allow the reactive gas which is in any case necessary for thedeposition of insulating coatings to enter the plasma zone through holesin the anode. This firstly leads to an intense ionization and excitationof this reactive gas and secondly in the immediate vicinity of theseholes the gas flow reduces the condensation of insulating coatings, sothat the arc discharges function for a much longer operating period. Thesecond effect can also be achieved by introducing an inert gas in placeof a reactive gas. A further increase in the operating time is obtainedif a vapor shield is placed between the plate-like anode and thevaporizer and/or by inclining the anode against the vapor jet a directvaporization of the anode surface is avoided. Another possibility foravoiding insulating coatings on the anode consist of heating the anodeto such an extent that as a result of revaporization no insulatingcoatings can be deposited. In the simplest case heating can take placeby making the anode from a heat-resistant material and maintaining it inthermally insulated manner, so that it is heated to the necessarytemperature by the electrons impacting from the hollow cathodedischarge. It is advantageous to build up the anode from individualbars, preferably of W, Ta or Mo and to place radiation shielding plateson the back of the anode bars. As a result of the heat conduction alongthe bars a wide area along the anode is heated and kept free, so thatthere is a widening of the plasma zone and in the case of severaljuxtaposed hollow cathodes a continuous, coating-free anode is obtained.As a result of the radiation shielding plates, for a given arc capacitya maximum anode bar heating is obtained. If in the case of extremelyhigh vaporization rates and particularly critical vaporization materialsa coating of the anode still occurs, it is then appropriate toadditionally heat the anode by current passage. In this case it isnecessary to make the current flow through the anode in such a way thatthe external magnetic field caused by the current flow does notinadmissibly influence the discharge arc.

According to an advantageous development the current is allowed to flowin meandering manner through parallel-arranged heating rods, preferablymade from W, Ta or Mo and behind the same are provided radiationshielding plates. According to another advantageous development theanode is built up from two plates or rod lattices successively arrangedin the arc direction and through which in both cases current flows inopposite directions. It is also appropriate to make the contacting ofthe anode and the anode leads such that also the outflowing arc currentproduces no disturbing magnetic field in the vicinity of the hollowcathode arc.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in greater detail hereinafter relative to twoembodiments and the attached drawings, wherein show:

FIG. 1 A section through a vaporizer for planar substrates with directinjection of the electron beam.

FIG. 2 A section through a vaporizer with a magnetic trap for strip-likesubstrates.

DETAILED DESCRIPTION OF DRAWINGS

The apparatus according to the invention shown in FIG. 1 comprises avaporizer 1, an electron gun 2 arranged in inclined manner above it andthe plate-like substrate 3 placed above the vaporizer 1 and which is tobe coated and is moved perpendicular to the paper plane over thevaporizer 1. The electron gun 2 is so flanged to the not shown vacuumchamber that the electron beam 4 strikes in inclined manner on thevaporizing material 5 located in the vaporizer 1 and in known manner canbe deflected onto the surface. The vaporizing material 5 is Al₂ O₃.Above and on either side of the vaporizer 1 are provided facing polepieces or shoes 6 in such a way that a magnetic field 7 extendingperpendicular to the paper plane is obtained and whose field lines aremainly parallel to the substrate 3. The maximum field strength in thecentral and lower area of the magnetic field 7 is 2 kA/m. In the upper,inhomogeneous marginal area of the magnetic field 7 is produced a zoneof high density plasma 8 by means of several juxtaposed hollow cathodes9, which are located in openings of the left-hand pole piece 6. (Onlyone hollow cathode 9 is shown, the others being positioned parallelthereto upstream and downstream of the drawing plane.) On the inside ofthe right-hand pole piece 6 is provided a watercooled anode 11 inclinedagainst the vapor beam 10 and elongated in accordance with the number ofhollow cathodes 9. The anode 11 has holes 12 from which reactive gas O₂is introduced from the back of the cathode 11 into the area of the highdensity plasma 8. The anode 11 is fixed by means of insulators 13 and isconnected in the conventional way to the power supply means for thehollow cathodes 9. From the hollow cathodes 9 burn parallel dischargearcs 14 to the anode 11 and which are curved towards the substrate 3.These discharge arcs 14 form the area of the high density plasma 8. Sothat in spite of the limited lateral extension of the tubular dischargearcs 14 there is a uniform plasma action over the entire substrate,during the coating said substrate is moved perpendicular to thedischarge arcs 14.

FIG. 2 shows a per se known vaporizer for coating plastic films 15,which pass over a cooling roller 16 through the vaporizing zone. Thevaporizing material 5 is a tube of SiO_(x), which constantly rotatesabout its longitudinal axis. The electron beam 4 produced with a notshown electron gun is injected horizontally into the vaporizing zone andis so bent and deflected that it is moved linearly on the tube parallelto its longitudinal axis and consequently a vaporizing line 17 isformed. To produce the necessary magnetic field a per se known magnetictrap 18 is so positioned that a magnetic field 7 is formed between thepole pieces 6 by the magnetic coils 19. The magnetic field 7 has amaximum field strength of 5 kA/m, so as to ensure that the electronsbackscattered on the vaporizing material 5 do not reach theheat-sensitive film 15. The tubular vaporizing material 5 extends overthe entire width of the film 15 to be coated. The uniform vaporizationover the entire film width is ensured by programme-controlled deflectionof the electron beam 4 along the vaporizing line 17 on the surface ofthe vaporizing material 5.

The magnetic field 7 produced by the pole pieces 6 has at least the sameextension perpendicular to the drawing plane. Several hollow cathodes 9are juxtaposed over the film width above the left-hand pole piece 6. Bymeans of suitable coils 20 an alternating magnetic field is produced ateach hollow cathode 9 and this leads to a fan-like deflection of theparticular discharge arc 14 parallel to the film face, i.e.perpendicular to the drawing plane.

The reciprocal spacing of the juxtaposed hollow cathodes 9 andconsequently their number per film width is dependent on therequirements made on the uniformity of the plasma action. The individualdischarge arcs 14 fanned out perpendicular to the drawing plane andwhich mutually overlap burn to the anode 11 located on the opposite polepiece 6 and extending over the entire film width. The anode 11 comprisesseveral, grid-like, thermally insulated fixed tungsten rods 21 behindwhich is positioned a heating beam-reflecting radiation shielding plate22. The tungsten rods 21 are fixed by not shown insulators to the polepiece 6 and connected to the power supply means for the hollow cathode9. The fan surface of the anode 11 is inclined against the direction ofthe vapor jet and is protected against direct vaporization by a vaporshield 23.

In all cases suitable means must be provided for ensuring that theplasma does not influence the electron beam 4 through by-passing orgetting round the magnetic field 7. Shields 24 are provided for thispurpose and are so positioned that electrons or ions from discharge arcs14 cannot in other ways come into the vicinity of the electron beam 4.

What is claimed is:
 1. An apparatus for plasma-assisted high rateelectron beam vaporization having a vaporizer containing a vaporizingmaterial and an associated electron gun producing an electron beam, aswell as a plasma source for producing a high density plasma, theapparatus comprising:a magnetic field-generating device having polepieces producing a magnetic field, said magnetic field-generating devicebeing positioned between the vaporizer and the substrate such that ahigh plasma density area produced by a plasma source located in animmediate vicinity of the substrate to be coated is separated from thevaporizing material and the electron beam acting on the vaporizingmaterial by said magnetic field;wherein said magnetic field is mainlyoriented parallel to the substrate, said high plasma density area beingguided along marginal field lines of the magnetic field in an arc curvedtowards the substrate.
 2. Apparatus according to claim 1, wherein saidmagnetic field-generating device is constructed and positioned such thata maximum field strength of the magnetic field between the high plasmadensity area and the vaporizer or the electron beam is 1 to 10 kA/m. 3.Apparatus according to claim 1, wherein the magnetic field-generatingdevice is a magnetic trap for shielding backscattered electrons. 4.Apparatus according to claim 1, wherein as the plasma source and as afunction of a width of the substrate to be coated, at least one hollowcathode is placed between one of said pole pieces of the magneticfiled-generating device and the substrate, and that on an inside surfaceof another of said pole pieces which faces said one pole piece, an anodeis positioned, said anode having an elongated length corresponding witha spatial arrangement of a number of hollow cathodes.
 5. Apparatusaccording to claim 4, wherein said number of hollow cathodes are locatedin openings of the one-pole piece.
 6. Apparatus according to claim 4,wherein at the location of an arc exit from the hollow cathodes, coilsare provided for generating an alternating magnetic field for a fan-likedeflection of an arc parallel to a surface of the substrate. 7.Apparatus according to claim 4, wherein the anode belonging to thehollow cathodes is inclined.
 8. Apparatus according to claim 4, whereina vapor shield is fitted to the underside of the anode.
 9. Apparatusaccording to claim 7, wherein the anode is a water-cooled metal plate.10. Apparatus according to claim 9, wherein the anode is a water-cooledmetal plate.
 11. Apparatus according to claim 7, wherein holes arearranged in the plate-like anode for introducing one of reactive gas andinert gas.
 12. Apparatus according to claim 9, wherein holes arearranged in the plate-like anode for introducing one of reactive gas andinert gas.
 13. Apparatus according to claim 4, wherein the anode is madefrom heat resistant material, if fixed in a heat insulating manner tothe other pole piece, and that radiation shielding plates are located ona back of the anode.
 14. Apparatus according to claim 7, wherein theanode comprises heat resistant bars.
 15. Apparatus according to claim 8,wherein the anode comprises heat resistant bars.
 16. Apparatus accordingto claim 7, wherein the anode is additionally heated by current passage,and wherein power supplies for the heating are positioned inside andoutside the anode in such a manner that external magnetic fieldsproduced by a current flow are substantially compensated in a vicinityof the discharge arc.
 17. Apparatus according to claim 8, wherein theanode is additionally heated by current passage, and wherein powersupplies for the heating are positioned inside and outside the anode insuch a manner that external magnetic fields produced by a current floware substantially compensated in a vicinity of the discharge arc. 18.Apparatus according to claim 2, wherein the maximum field strength isbetween 2 to 5 kA/m.
 19. Apparatus according to claim 9, wherein theanode is made of copper.
 20. Apparatus according to claim 14, whereinthe anode consists of one of the group of tungsten, tantalum ormolybdenum.
 21. An apparatus according to claim 1, wherein saidplasma-assisted high rate electron beam vaporization has evaporationrates of from a few μm/s to 50 μm/s.